Add copy/paste

[AlexBxl]

Overview

Copying and pasting nodes should probably be done through a text format like JSON for easier interop with the outside world. For the same reasons it probably makes sense to use the system clipboard and not something internal.

Cut ≠ Copy+Delete

Cut might need special handling for situations like when the user cuts some content and the app crashes, the cut content should at least be recoverable.

Inputs & Undo

Cut, Copy & Paste etc. should be undoable actions. Just like with undo, because browser inputs handle cut/paste by themselves, we need to figure out how to cooperate with it.

Alt/Option+Drag

This method of copying is very very convenient for maintaining flow when building graphs.

Duplicate

Copy/Paste functionality will make duplication easy to implement, something like Copy+Paste+Move (Move if we want to offset the duplicate), though this should probably be a separate ticket.

Drag & Drop

Using JSON helps to implement things like drag & drop loading of graphs, but we need more metadata. Also probably a separate ticket.